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CEMS STUDY OF Fe60Al40 THIN FILMS
A. Fnidiki, J. Eymery
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplkment au no 12, Tome 49, dhcembre 1988
CEMS STUDY OF Fe60A140 THIN FILMS
A. Fnidiki and J. P. EymeryLaboratoire de Mitallurgie Physique, U.A.131 du C.N.R.S., 4 0 avenue du Recteur Pineau F-86022 Poitiers Cedex, France
Abstract. - The coevaporation technique offers the possibility to prepare Fe60A140 thin films which behave ferromag- netically at 293 K. They show nearly same magnetic properties as cold worked or ion-implanted bulk FesoAlro. On the contrary, their behaviour strongly differs at higher temperatures so that the precipitation of iron rich phases is inferred from CEMS analysis.
In a recent paper 111, it has been shown that magnetic FesoAl40 films could be prepared by coe- vaporation. This result is important because bulk FesoA140 having the L20 superstructure is parama- gnetic at 293 K and previously it could be rende- red ferromagnetic a t this temperature only under cer- tain conditions, namely after cold working [2] or ion- implantation 131. These treatments were assumed t o create atomic disorder so that a given Fe atom had a good probability to be surrounded by at least three or four Fe atoms which is necessary for him t o bear ma- gnetic moments. We describe here further results on the magnetic properties of coevaporated Fe60A140 thin films (200 nm) deposited on a Si substrate. Special em- phasis will be placed on the thermal behaviour of the films which showed striking features. The magnetic properties were chiefly evaluated with the Mossbauer effect working in scattering geometry (CEMS); addi- tional informations were obtained with a polar Kerr effect measurement systems.
The films were deposited on a Si substrate by coe- vaporation. Two E-beams installed in a high va- cuum chamber were used; the starting pressure was I x lo-* Torr and during deposition it was maintai- ned a t about 2
-
3 xlo-'
Torr; 'the deposition rate of each element, typically 0.2-0.3 nm/s, and the thick- ness of the layer were monitored by two quartz oscil- lators. Concerning the CEMS technique, the 7.3 keV conversion and 5.6 keV Auger electron spectra were taken using a source of 50 mCi "CO in rhodium, a pro- portional counter with a He-5 % CH4 gas flow and a conventional Mossbauer spectrometer; the counter has been recently described in details in reference [4]. The analysis of the spectra was performed by summing a central Lorentzian line and a distribution of hyperfine fields (Window method [5]). The magnetization was determined using a polar Kerr effect measurement de- vice, the laser of which had a wavelength of 632.8 nm. The 293 K electron spectrum of an as-coevaporated thin film is presented in figure la. The average hy- perfine field H is found to be 218 kOe while the per- centage of magnetic iron is 74 % (see Tab. I). TheseFig. 1. - Room temperature GEM spectra: (a) as- coevaporated, (b) annealing temperature = 573 K, (c)
773 K, (d) 873 K.
results are in good agreement with those determined on both implanted [3] and crushed [2] bulk samples. Figure 2 shows the variation in Kerr rotation angle
OK
as a function of applied magnetic field. Saturation is reached forOK
= 13' and 47rM = 6.4 kOe; assuming a density of 6.02 g . ~ m - ~ , the magnetic moment is found to be a = 84.5 emu/g, a result which is also in agree ment with the value determined on implanted samples by ferromagnetic resonance [6]. From the above data it is quite satisfactory to note that the three kinds of sample preparation namely cold working, ion- implan- tation and coevaporation, lead to nearly same magpe- tic properties.Fig. 2. - Kerr rotation angle versus magnetic field at
293 K.
C8 - 1736 JOURNAL DE PHYSIQUE We report now the thermal behaviour of the films
as determined by CEMS. The sample under study was annealed for one hour under a Torr vacuum at the required temperature in the range 293-873 K and then the CEM spectrum was taken a t 293 K. The spectra are presented in figures lb, c, d while the correspon- ding Mossbauer parameters are listed in table I.
Table I. - Mossbauer parameters. Left and right sides correspond to field distribution and paramagnetic cen- t m l line respectively: H = average hyperfine field,
U H = standard deviation, b = relative intensity of Hnes
2 and 5 , ff = ferromagnetic fraction, IS = isomer shift with respect to a-iron,
I?
= half line width. The large values ofr
determined for spectra l a ,5
indicate the paramagnetic fraction is also distributed.First, we have to point out that the spectra unam- biguously show a magnetic fraction (ff) up to at least 773 K; a t this temperature ff = 100 % so that no para- magnetic iron is remaining. A paramagnetic spectrum is only obtained after annealing a t 873 K; the most probable reason for this change is the diffusion of Si from the substrate h t o the film. A strong difference
then appears with respect t o both cold worked and ion-implanted bulk FesoAlro for which the magnetic in- teractions disappeared at temperatures slightly higher than 293 K [7]; in this case, L20 atomic reordering was assumed to take place, thus leading to paramagnetic spectra. On the contrary, the hyperfine field of the coevaporated films increases with increasing tempera- ture up t o 301 kOe, a value slightly higher than that of D sites in ordered Fe3A1, i.e. 290 kOe. This varia- tion in field indicates the presence of iron-rich domains, the relative abundance of which increases up to 100 % at 773 K. Atomic ordering of L20 type, if any exists,
(K) 293 after I h at 573 after I h at 873
may contribute t o the paramagnetic fraction but from the above result it is clear that the basic phenomenon is rather new phase precipitation than L20 ordering. This behaviour is first surprising; however, it is to be recalled that the coevaporation technique leads to me- tastable random alloys exhibiting local fluctuations of composition because the evaporation rate from each source cannot be maintained rigorously constant. In our case, the Mossbauer spectra suggest that at 773 K, significant atomic motion becomes possible enabling the transformation of metastable FesoAlro into one or several phase(s) of higher stability close to Fe3A1 and a-Fe. Finally it is noteworthy that Godbole et al. 181 have already observed same trend in ion-beam-mixed FergiAl46 samples.
As a conclusion we. have prepared thin films of
a
O H b ff (kOe) ( k O e ) (%) 218 51 1.1 74 290 18 2 46 301 15 0.45 100-
-
--FesoA140 by coevaporation, the magnetic properties of which at room temperature appear to be nearly the same as those of bulk material chemically disordered
IS
r
( m m / s ) ( m m f s ) 0.15 ' 0.5 0.10 0.5 - - 0.15 0.26either by cold working or by ion-implantation. Howe- ver, a t temperatures higher than 293 K, i.e. in the 573-773 K range, the films show a tendency towards new iron rich phase precipitation instead of L20 orde- ring as expected from the bulk behaviour.
[I] Fnidiki, A., Eymery, J. P. and Junqua, N., Solid State Commun. 63 (1987) 549.
121 Huffman, G. P. and Fisher, R. M., J. Appl. Phys. 38 (1967) 735.
[3] Fnidiki, A., Eymery, J. P. and Delafond, J., J.
Magn. Magn. Mater. 40 (1983) 130.
[4]' Bodin, D. and Eymery, J. P., Nucl. Instrum. Me- thods B 16 (1986) 424.
[5] Window, B., J. Phys. E 4 (1971) 401.
[6] Krishnan, R., Suran, G., Eymery, J. P., Rivihre, J. P. and Fnidiki, A., Phys. Lett. A. 121 (1987) 43.
[7] Fnidiki, A., Bodin, D. and Eymery, J. P., -Hyp. Inter. 29 (1986) 1179.
